Most golf balls sold in the U.S. are listed on the conforming list of the United States Golf Association (USGA). Several specifications have been established by the USGA and a golf ball must meet certain test criteria relating to these specifications for weight, size, initial velocity, overall distance (carry and roll), and spherical symmetry. For acceptance by the USGA, a golf ball must not weigh more than 1.620 ounces, must have a minimum diameter of 1.680 inches, must have a maximum initial ball velocity of 250 feet per second (plus a maximum 2% tolerance) as measured on a standard USGA ball testing machine, and must have an overall distance maximum of 317 yards (plus a maximum 3 yard tolerance) as measured by the USGA overall distance test procedure. Further, the ball must not be designed, manufactured, or intentionally modified to have properties that differ from those of a spherically symmetric ball. The USGA tests for symmetry by inspecting the statistical deviation of the overall distance test data (distance variation and flight time variation) when the ball is struck from various aspects.
A typical golf ball is either of the “wound” or “molded” type. Since molded golf balls are cheaper to produce, virtually all of the golf balls currently sold are molded. Due to their price, most golf balls sold today are two-piece polymeric balls with polybutadiene cores. The process of making this type of ball includes compression molding the polybutadiene into a solid core of various diameters and then injection molding a cover onto this core. Dimples are included in the die and so the second step of injection molding produces a nearly finished golf ball, with clean up and painting typically being performed to finish the ball.
Most development of new golf balls is based on this two-piece architecture with the solid polybutadiene core, but adds various layers between the core and the outermost cover. Between two and six cover layers can be added. Some balls use an injection molded core or mantle layer (between the core and cover layers), but the majority of even the bestselling tour balls employs a compression molded polybutadiene core.
Most commercially available golf balls are made of nonmetallic rubbers and plastics, such as elastomers, ionomers, polyurethanes, polyisoprenes, nylons, and other similar materials. In recent years, however, golf balls with hollow metal cores have appeared in the market. This design takes advantage of the high stiffness of the metal core (when compared to the stiffness of typical golf ball materials) to produce a golf ball that has simultaneous characteristics of high accuracy (less hook and slice) as well as improved putting characteristics. These types of golf balls still have some shortcomings, including hard feel and a small loss of distance compared to more typical molded balls discussed above.
Simply using a metal in part of the golf ball is not sufficient to significantly increase the stiffness of the golf ball. Such designs suffer from several shortcomings including a hollow metal sphere design that is not durable enough to withstand impact forces when struck by a club. This can lead to permanent distortion.
Other recent developments do not provide a significantly higher stiffness in the core.
Accordingly, there is an ongoing and unmet need for golf balls having a high stiffness core that do not exhibit the shortcomings of existing golf balls.